Neopentyl glycol (NPG, IUPAC name 2,2-dimethyl-1,3-propanediol) is an important industrial intermediate used in the synthesis of polyesters, especially resins for airplane or boat manufacturing. The neopentyl structure provides excellent resistance to light, heat, and hydrolysis. NPG is also being used in a variety of formulations in varnish coatings, synthetic lubricants, and plasticizers.
Methods for producing NPG is well-known in the prior art. For example, U.S. Pat. No. 4,855,515 (Eastman Kodak Company) described a two-step method for producing NPG. The first step is aldol condensation reaction of formaldehyde (FD) and isobutylaldehyde (IBD) to form an intermediate product hydroxypivaldehyde (HPD), which is then hydrogenated to NPG in the second step.

It is also known from the prior art that the aldol condensation reaction is typically performed in the presence of a basic catalyst, while the hydrogenation reaction can be conducted with a typical hydrogenation catalyst such as Raney Nickel, as disclosed in U.S. Pat. No. 2,818,443 (Celanese Corporation of America), U.S. Pat. No. 3,920,760 (Eastman Kodak Company), U.S. Pat. No. 5,841,002 (Davy Process Technology Limited), U.S. Pat. No. 6,201,159 (LG Chemical Limited), U.S. Pat. No. 6,268,539 (Nan Ya Plastics Corporation), and U.S. Pat. No. 8,394,998 (Oxea GmbH). The use of other catalysts have also been disclosed; for example, U.S. Pat. No. 5,395,989 (Mitsubishi) teaches the use of CuO—ZnO—ZrO mixture as catalyst, while U.S. Pat. No. 5,532,417 (Aristech Chemical) suggests copper chromite.
If a strong alkali such as sodium hydroxide or calcium hydroxide is used, the hydrogenation step can be eliminated by using excess formaldehyde to convert HPD to NPG via cross-Cannizzaro reaction (Barnicki, 2012), resulting in essentially a one-step method to produce NPG from FD and IBD, in which the strong alkali is both a catalyst and a reactant.

Although this method only requires one reactor, it produces a formate salt such as sodium formate as a by-product, which is undesirable for commercial scale manufacturing. Furthermore, a more involved separation process will be required to recover NPG.
The use of a sodium compound as aldol condensation catalyst in the two-step method also leads to the need of a complex separation process, because the sodium compound has to be completely removed prior to hydrogenation to avoid poisoning of the hydrogenation catalyst. For example, U.S. Pat. No. 3,920,760 (Eastman Kodak Company) describes a separation process involving filtration, decantation, and coalescence in order to remove sodium compounds after aldol condensation. Therefore, it is more desirable to use a catalyst that does not contain sodium. For example, U.S. Pat. No. 3,808,280 (BASF) teaches about the use of a tertiary alkylamine as the aldol condensation catalyst. Since an aqueous formalin solution is normally used as the source of FD, while IBD is not miscible with water, the reaction system consists of two liquid phases, and the tertiary alkylamine also acts as a phase transfer catalyst which facilitates the reaction. U.S. Pat. No. 2,818,443 (Celanese Corporation of America) proposes the use of a weak basic ion exchange resin (IER), which can be easily separated from the intermediate product, thereby minimizing separation effort. However, an extra solvent such as methanol or other aliphatic alcohols has to be added to homogenize the solution. Another alternative to avoid liquid-liquid phase separation is to use paraformaldehyde instead of formalin as the source of fomaldehyde so that water is not present in the system, as disclosed in U.S. Pat. Nos. 5,144,088, 5,146,012, 5,185,478, and 5,532,417 (Aristech Chemical Cooperation).
In order to obtain NPG in a highly pure form, it is required to remove by-products that are generated from either the aldol condensation or hydrogenation reaction. U.S. Pat. No. 6,201,159 (LG Chemical Limited) discloses a process for the production of NPG that involves extraction and distillation steps to isolate HPD prior to hydrogenation, and a saponification step followed by another set of extraction and distillation steps to obtain highly pure NPG after hydrogenation. Saponification with caustic is useful to remove ester impurities from NPG, but the caustic-containing mixture is unstable at high temperatures, so that steam sublimation or film evaporation is usually required, as suggested in U.S. Pat. No. 4,935,555. The complexity of the separation process potentially leads to high capital cost and energy consumption. A simpler separation process prior to hydrogenation has also been suggested. For example, U.S. Pat. No. 4,036,888 (Societe Chimique des Charbonnages) and U.S. Pat. No. 5,144,088 (Aristech Chemical Cooperation) suggest the use of crystallization for isolating HPD after the aldol condensation reaction. However, this alternative also requires a relatively high energy consumption since the HPD crystals have to be melted before the hydrogenation step.
Accordingly, further developments and improved processes are greatly needed.